1. Field of the Disclosure
The invention relates generally to the field of electromagnetic evaluation formations in the earth's surface. More specifically, the invention relates to methods for determining electromagnetic attributes in specific formations in the subsurface to a relatively high lateral and vertical resolution.
2. Description of the Related Art
Exploration for and exploitation of petroleum resources are entering a new phase wherein many methods are utilized to develop an integrated understanding of potential and discovered reservoir rocks. One of the overall goals of an integrated strategy for exploration and exploitation is to reduce the risk, especially that associated with drilling in new basins or harsh, offshore environments. Some traditional geophysical exploration methods for oil and gas (seismic, gravity and magnetic) have been tuned to exploit indirect indicators of petroleum occurrences—that is, they best delineate the structures that are potential petroleum reservoirs. Such traditional methods are not, in the first instance, direct indicators of petroleum.
Seismic methods developed over the past 60 years or so are highly refined and have led to a significant reduction in drilling risk to the point where approximately one in six exploration wells is deemed successful. In the past decade, special seismic methods (4D/time lapse and attribute characterization) have been developed to further reduce the risk of drilling. The major limitation in conventional geophysical exploration interpretation is that the physics of the methods inherently identifies contrasts in macro physical properties (velocity, density and magnetism). Thus, conventional geophysical exploration is traditionally best suited to delineate structural traps. Such methods have been successful in finding the major structures worldwide that contain oil and gas.
The primary properties of sedimentary rocks that lead to direct indication of oil and gas have to do with the porosity and permeability of the sediments, i.e., the nature of the pore fluid, the percentage of the rock volume that is fluid-filled, and the migration characteristics of the fluid. Unfortunately, while some progress has been made in utilizing the second order effects of pore fluids on seismic velocity and bulk density of the rock formations, these effects are still subtle and are traditionally subject to substantial uncertainty, especially for deeper reservoirs as are often found in offshore basins.
In contrast to the acoustic and magnetic properties, the electrical properties of sedimentary rocks are almost entirely determined by the volume and nature of the pore fluids. Virtually all common rock-forming minerals in sediments are electrical resistors, for instance, quartz (SiO2) and mica, which are often used as electrical resistors in electronic microcircuits. Hence, because these rock-forming minerals are most often electrical resistors, the type of sediment (carbonate, clastic rock, or salt/anhydrite) has little impact on the bulk electrical properties of the rock and pore fluids. The electrical properties are determined almost exclusively by the amount and nature of the pore fluids. Furthermore, again unlike acoustic properties that vary only over a factor of 2 in the most extreme case (typically <10% or so), bulk electrical properties of sediments can vary by several orders of magnitude depending on the value of the porosity (0.1% to >20%) and the pore fluid (connate water, oil or gas). The noteworthy correlation is that while connate water is saline to some extent and thus substantially electrically conductive (compared to the rock forming minerals), petroleum fluids (i.e., oil and gas) are essentially non-conductors of electricity. This difference in conductivity leads to the potential to exploit this extreme property difference in a geophysical method that is a direct hydrocarbon indicator.
Those of skill in the art will recognize that significant literature exists pertaining to the electrical properties of sediments from both the petroleum well logging and mineral exploration fields. Empirical relationships have been developed that describe electrical resistivity compared to porosity and a large body of well log correlations to guide interpretation. These can be used to assist in determining the appropriate frequencies for any electromagnetic exploration method.
There are several electromagnetic techniques from the mineral sector repertoire that have recently been adapted for petroleum exploration. Prime among these is the use of towed dipole systems exploiting electromagnetic and magnetotelluric fields. These have found favor in both shallow water and deep marine settings. Typical electromagnetic marine surveys are extensively described in the literature and in an extensive listing of patents. The basic method involves a vessel which tows cables connected to electrodes deployed near the sea floor. The geophysical support vessel generates high power signals to the electrodes such that an alternating current of selected magnitude (magnitudes) and frequency (frequencies) flows through the sea floor and into the geological formations below the sea floor. Receiver electrodes are deployed on the sea floor at a range of offsets from the source electrodes and are coupled to a voltage measuring circuit. The voltages measured at the receiver electrodes are then analyzed to infer the structure and electrical properties of the geological formations in the subsurface.
Another well known technique for electromagnetic surveying of geological formations is known in the art as transient controlled source electromagnetic surveying. Typically an electric current, normally direct current (DC), is imparted into the seafloor. At a selected time, the electric current is switched off, switched on, or has its polarity changed (or one or more of such events occur in a coded sequence), and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface, near the water bottom or water surface. The structure of the subsurface is inferred by the temporal and spatial distribution of the induced voltages and/or magnetic fields. These techniques are described in various publications such as by Strack, K.-M., 1992, Exploration With Deep Transient Electromagnetics, Elsevier, 373 pp. (reprinted 1999).
These traditional techniques for electromagnetic surveying suffer from a number of problems. In traditional methods, low signal to noise ratios may make proper analysis of the electromagnetic survey difficult. Further, such methods may be deficient in that they provide a low resolution picture of the subsurface Earth structures, again making proper analysis problematic. Finally, such traditional methods often are difficult to focus on particular areas of the survey, such as areas that appear to be likely to contain petroleum bearing strata. It follows that there is a need to develop an electromagnetic surveying method that addresses such issues.